Corrosion resistance of electroless nickel plate



RESISTANCE or ELECTROLESS NICKEL PLATE Herbert K. De Long and Donald E. Ritzema, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Feb. 6, 1958, Ser. No. 713,566

2 Claims. (Cl. 117-71) CORROSION The invention relates to improving resistance to corrosion of nickel plate formed by the electroless nickel plating process. It more particularly relates to such improved resistance to corrosion of nickel plate formed according to the method described in application Serial Number (S.N.) 663,871, filed June 6, 1957, now abandoned, and in continuation-impart application thereof, S.N. 725,510, filed April 1, 1958.

Electroless nickel plating refers to the deposition of metallic nickel from an aqueous bath (bufiered to a suitable pH value) by the chemical reduction of nickel ions contained therein to metallic nickel in the presence of a hypophosphite, on the surface of a material immersed therein which catalyzes such chemical reduction. Metals which catalyze such chemical reduction are referred to generally as catalytic-basis metals. Prior to the invention described in U.S. application S.N. 663,871, these metals included iron, aluminum, brass, copper, tin, nickel, gold, silver, Monel metal, stainless steel, cobalt, chromium, paladium, vanadium, tungsten, and molybdenum. U.S. application S.N. 663,871 describes an improved bath and method of electroless nickel plating catalytic-basis metals generally and especially to so-plating magnesium and its alloys.

The nickel plate formed on catalytic-basis metals, and particularly on magnesium-base alloys according to S.N. 663,871; are bright, hard, and adherent. Although the plate thus formed is equal or superior in corrosion resistance to other nickel plate deposited by electroless methods from nickel-hypophosphite baths, corrosion does occur under severe conditions, e.g., during salt water exposure. A need,'therefore, exists for a method of improving the corrosion resistance of nickel plate prepared by the elect-roless method.

Accordingly, it is the principal object of this invention to provide a method of treating electroless nickel-plate on a catalytic-basis metal to lessen substantially the rate of corrosion of such plate. I

The practice of the invention is applicable to treating electroless nickel plated metal articles plated by any method which employs a plating composition containing hypophosphite and nickel ions at a suitable pH value.

For purposes of illustration, however, a flow diagram showing the electroless nickel plating process for plating a metal article, as described in S.N. 633,871 and S.N. 725,510, followed by the subsequent treatment of the so ice Patented Mar. 14,

2 V plated article to impart corrosion resistance thereto in accordance with the invention is set forth below.

/7l Metal article to be electroless nickel plated, e.g., :1 Mg. alloy Surface V conditioning Source of F-,

eg., KF

Water rinsing Source of Ni", e.g.,

NiCOa-Ni(OH):-4Hz0 Eleetroless Source of HaPOt", nickel plating e.g., N BHzPOg-HgO pH control agent Water e.g., HF or N H4011 rinsing Bufiering agent e. Corrosion resistant N83C6H507 ssino treatment according to the invention Water rinsing 7 Corrosion resistant electroless nickel E plated metal article can be between 0.05 and 24 percent of the weight of the bath. It is preferred that where chromic acid is used, it be present in an amount between about 0.1 and 5 percent and where the soluble chromate is used that the chromate compound be present in an amount between about Sand 15 percent of the weight of the bath. The temperature of the bath is not highly critical, any temperature between about 60 and 212 F. being satisfactory and a temperature between about and 212 F. usually ing used. The pH of the bath can be any value between 0.1 to 8 and is preferably between 0.5 and 5. The period of time of immersion of the electroless nickel plated metal piece in the bath of the invention can be between and 15 minutes although any time between about 0.5 minute and 60 minutes can be employed. A period of about minutes at about 212 F. is recommended.

The method described in S.N. 663,871 employs a novel aqueous bath containing between 1.6 and 49 gram ions of fluorine, between 2.5 and 70 gram ions of nickel, and between 3 and 165 gram ions of hypophosphite per liter of the bath.

The bath usually contains between 1.6 and 7 gram ions of fluorine and preferably about 3.5 gram ions per liter. They are usually provided in the bath by addition thereto of aqueous hydrofluoric acid solution of about 50 percent of HF by weight. To provide a common ion effect and thereby to suppress the ionization of HF as desired for pH control, KF, NH F or NaF is usually added. To obtain the common ion effect and limit the ionization of the HF for the purpose of the pH control, an amount of KF between about 1 and 40 grams and preferably about 20 grams per liter of the bath serves as an example. Control of the pH value is also effected by further addition of a salt of an organic acid such as sodium or potassium citrate, succinate, tartrate, lactate, or phthalate.

The nickel ion concentration employed in S.N. 663,871 can be between 2.5 and 70 gram ions per liter and is usually between 2.5 and 20 gram ions per liter, about 5 grams per liter of solution being most desirable. Basic nickelous carbonate, 2NiCO -3Ni(OH) -4H O, is the preferred source of nickel ions in the bath.

A water-soluble hypophosphite, e.g., NaH PO -H O, is employed to provide the hypophosphite ions in'an amount between 3.0 and 165 gram ions per liter, preferably between 3.0 and 36.0 gram ions, and usually about 11.9 gram ions per liter. To provide 11.9 gram ions of hypophosphite ions, for example, 20 grams of NaH PO -H O are employed.

The surface of a catalytic-basis metal to be plated according to the method described in S.N. 663,871, must be free of contaminants such as oxides, oil and bits of foreign material. Conventional cleaning procedures can be employed for the removal of such contaminants among which are sand-blasting, wire-brushing, or immersion in chemical cleaning baths.

After cleaning the metal surface, if found necessary, the metal to be plated is immersed in the bath described in S.N. 663,871 for at least 2 minutes and for as long a time as is necessary to obtain a plating of satisfactory thickness. The temperature of the bath is between 90 and 212 and preferably is between 160 and 212 F., and is usually about 180 F.

The article thus plated according to S.N. 663,871, is removed from the bath. water-washed to remove adhering bath, and dried. A hard, bright, adherent, electroless plate is thus obtained. If desired, the brightness of the plate may be enhanced by bufiing according to known practice. The desired thickness of the plate thus formed is anywhere between 0.0001" and 0.00

Among the catalytic-basis metals which have been successfully plated according to the method described in S.N. 663,871 are the magnesium-base alloys found under the following designations: sand castings, ASTM B8057T, containing the following: EK3OA, EZ33A, AZ63A, HZ32A, HK31A, AZ81A, and AZ91C; extrusions, ASTM BlO7-'-57T, including: ZKGOA, AZ80A, AZ61A; die castings, ASTM B9442, illustrated by AZ91A; sheet, ASTM D90-56T, which includes AZ31A and MIA; US. Air Force specified alloys designated Mil-M-26075 illustrated by HK31A; and two experimental alloys, one containing 1 percent zinc, 0.2 percent rare earth metals, and the balance-magnesium, and the other containing 2 percent thorium, l'percent manganese, and'the balance magnesium.

An example of electroless nickel-plating panels of magnesium-base alloy according to S.N. 663,871 is set out below: A series of panels 2" x 1" x 0.040 in size and having a composition of 3 percent Al, 1 percent Zn,

and the balance Mg were prepared and immersed in a bath made up according to Example 1 of SN. 663,871 until a nickel plating of 0.0005 was formed. The plated panels were then removed, washed, and dried.

A series of baths having the compositions set out in the table below were prepared for use according to the in stant invention. Thereafter, a panel of the series of panels which had been nickel-plated according to S.N. 663,871 was placed in each of the baths thus prepared, according to the method of the invention. One panel designated Blank, was not treated but was also subjected to the salt spray test for comparative purposes. The conditions of the treatment and the corrosive effects of a salt spray on each panel thus treated are also set out in the table below:

Table Percent Time. Temp., Corrosion Aqueous Solution Used pH Min. I Resulting From Salt Spray Test 10 percent NazCrzO7-2HqO 4.2 10 75 9 10 percent NazCIzOrZHzO 4.2 10 212 1 10 percent N82Crz07-2Hz0 4. 2 30 212 1 10 percent NazGlzOt-ZHzO 8.0 10 212 7 0.5 percent 0103 l. 7 10 212 ll) 24 percent Cr0+0.24 percent H2804 0.1 10 100 G 18 percent Na2CIz07-2H20-l-18 percent; HNO; 0. 5 1 75 4 Blank: No Treatment 15 cent salt spray at F. for 24 hours according to the military test Mil-E-5272A which is also designated ASTM test D1 17-54T. From the above table, it is evident that the corrosion resistance of electroless nickel plated magnesium alloy panels is definitely improved by treating such plated panels according to the instant invention.

The bath as used in the instant invention is effective on only electroless plate and is not applicable to electroplate formed by the employment of electric current. To show the difference in the corrosion resistance effected by treatment according to the instant invention on electroless nickel plate and that obtained when electroplate is subjected to a similar treatment, a series of magnesiumbase alloy panels of the type employed in the examples above was electroplated according to conventional practice. This consisted essentially of making a piece of nickel metal, the anode, and the magnesium panel to be plated, the cathode, in an aqueous solution of a nickel salt until a sufficiently thick plate of nickel Was electrodeposited, removing the panel "from the aqueous nickel salt, waterrinsing,'and drying. The electroplated panels so prepared were then divided into two sets. The first set was immersed in the 10 percent aqueous solution of (the same solution as that used in the first four tests of the table above) for 10 minutes at 212 F.; the second set of panels was not treated. Both sets were thereafter subjected to the salt spray test described above. No significant difference in corrosion could be detected between the untreated panels and. those treated by immersion in the bath.

Although the examples in the table above have been directed toward electroless nickel-plated magnesium alloy articles plated according to the method of SN. 663,871, it is understood that the corrosion resistance imparted by the method of the instant invention is applicable to any electroless plate or coating formed by chemical deposition of nickel from a bath containing nickel ions and hypophosphite ions on any catalytic-basis metal.

The foregoing description shows that marked improvement in resistance to corrosion is obtained when nickel plate deposited according to the electroless method is treated according to the invention.

Having described the invention, what is claimed and desired to be protected by Letters Patent is:

1. The method of increasing the corrosion resistance of nickel plate chemically deposited from an aqueous solution containing nickel and hypophosphite ions at a pH of between 3.5 and 7 on a magnesium-base alloy article immersed therein which comprises thereafter immersing the thus nickel-plated metal magnesium-base alloy article in an aqueous solution having a pH between 0.5 and 5 and containing a chromium compound selected from the class consisting of chromic acid and sodium dichromate, said chromic acid being employed in an amount between 0.1 and 5 percent and said sodium dichromate between 5 and 15 percent by weight of the bath.

2. In the method of producing a nickel plated magnesium-base alloy article having a deposition of nickel formed thereon by immersing said article in an aqueous 6 solution containing nickel ions and hypophosphite ions and having a pH value of between 3.5 and 7.0, the improvement consisting of immersing the thus nickel plated magnesium article in an aqueous bath having a pH value of between 0.1 and 8.0 and containing a chromium compound selected from the class consisting of chromic acid, soluble chromates, and soluble dichromates in an amount of between 0.1 and 5.0 percent by weight when chromic acid is employed and between 5.0 and 15.0 percent by weight when soluble dichromates and soluble chromates are employed, at a temperature between about 60 and 212 F., removing the thus-treated plated article from the bath, and washing with water.

Giesker et al. Apr. 9, 1957 Wrotnowski Mar. 29, 1960 

1. THE METHOD OF INCREASING THE CORROSION RESISTANCE OF NICKEL PLATE CHEMICALLY DEPOSITED FROM AN AQUEOUS SOLUTION CONTAINING NICKEL AND HYPOPHOSPHITE IONS AT A PH OF BETWEEN 3.5 AND 7 ON A MAGNESIUM-BASE ALLOY ARTICLE IMMERSED THEREIN WHICH COMPRISES THEREAFTER IMMERSING THE THUS NICKEL-PLATED METAL MAGNESIUM-BASE ALLOY ARTICLE IN AN AQUEOUS SOLUTION HAVING A PH BETWEEN 0.5 AND 5 AND CONTAINING A CHROMIUM COMPOUND SELECTED FROM THE CLASS CONSISTING OF CHROMIC ACID AND SODIUM DICHROMATE, SAID CHROMIC ACID BEING EMPLOYED IN AN AMOUNT BETWEEN 0.1 AND 5 PERCENT AND SAID SODIUM DICHROMATE BETWEEN 5 AND 15 PERCENT BY WEIGHT OF THE BATH. 